It is a common feature on harvesters such as windrowers to have a hydraulic lift system that enables the operator to raise the header into a transport position for turning around at the end of a row. On self-propelled machines, it is also common practice to provide a hydraulic flotation system that carries much of the weight of the header as the machine moves across a field so that the header only lightly skims the ground and can easily be pushed upwardly by rises in the ground to the extent necessary to clear such changes in contour.
In the past, self-propelled windrowers have typically been provided with a pair of single-acting cylinders for lifting opposite ends of the header. The two lift cylinders are plumbed in parallel and each has a single hydraulic line supplying one end of the cylinder with pressurized fluid, the other end being vented to the atmosphere. As the machine moves through the field of standing crop, the weight of the header is partially borne by the lift cylinders, which provide a solid "floor" or "base" that prevents the header from swinging down below a certain height. The degree to which the cylinder remains extended thus determines the cut-off height for the header, which can float up from the cut-off height due to the assistance of the flotation cylinders when uneven terrain is encountered.
Problems can arise with this arrangement in rough fields, however. Because the lift cylinders are single-acting, there is nothing pushing down on the pistons of the cylinders as the machine moves across the field. Consequently, the pistons have a tendency to bounce when rough spots are encountered by the tractor, causing the header to bounce as well. This can produce an uneven, ragged cut.